Our objective is to understand the mechanisms responsible for the formation, maintenance and regression of the structure of corpora lutea and the mechanisms responsible for the initiation, maintenance and termination of their function. At present, the structure and function of corpora lutea appear to be regulated by factors from the pituitary gland, the uterus and the opposite ovary. We have examined the relationship of the number of corpora lutea and the weight of luteal tissue on the rate of progesterone secretion from the ovary of the pig. With this information, we have developed statistical methods which will permit us to study validly the influence of extra-ovarian substances on luteal structure and function. Our technological objective is to develop radioimmunoassay methods for the simultaneous measurement of luteinizing hormone (LH), follicle simulating hormone (FSH) and prolactin (LTH) and the gonadal steroids - estrone, estradiol and progesterone - in both ovarian venous and peripheral serum. With these methods (some of which we have developed) we will investigate the complex mechanisms which regulate corpora lutea in the pig. We have developed restraining stalls for adult pigs (to which they readily adapt) and the surgical techniques required to provide an animal that can be employed in experiments while standing, unanesthetized, unstressed and unconscious of being injected or of being sampled for blood. Catheters have been implanted in the vena cava, the ovarian vein and the uterine vein. Correlations of vena caval, ovarian venous and uterine vein concentrations of gonadotropins and steroids will provide a complete picture of functional, structural and temporal relationships of hormones and glandular functions.